Optical networks (NWs) employing a technique such as a WDM (Wavelength Division Multiplexing) technique are being constructed in large-scale backbone networks operated by carriers. The WDM technique is a technique that accommodates traffics of a plurality of users in optical signals having different wavelengths between nodes connected by optical fibers, and multiplexes a plurality of optical signals into an optical transmission line for a large-capacity transmission.
In the WDM technology, an optimal modulation scheme is selected depending on a transmission distance and the like, and as a result, a mixture of different modulation schemes such as QAM (Quadrature Amplitude Modulation), DMT (Discrete MultiTone Modulation), QPSK (Quadrature Phase Shift Keying), RZQPSK (Return to Zero QPSK) and the like may be employed in the WDM technology.
In the WDM technology, for example, when a section that causes an error is specified in an occurrence of the error, the cause section in an optical transmission line may not be evaluated only with a simple measurement of OSNR (Optical Signal to Nose Ratio) on the optical transmission line. Therefore, the cause section in the optical transmission line is evaluated by measuring OSNR for each transmission section.
Related technologies are disclosed in, for example, Japanese Laid-Open Patent Publication No. 09-153865.